https://www.brennancenter.org/our-work/research-reports/vote-safely-2026
That resource is something voters, and election judges in particular, should read.
If nothing else, there is phone information for an "election protection hotline" to call if any intimidation show, or action, is noticed at a polling place. And a checklist of info to provide. Presumably the local county sheriff's office can be called, to give notice of an improper polling place situation, and ICE or whoever can be told to leave, and should.
Beyond that, check the Brennen Center resource itself, as linked to above.
In particular, election judges should pay attention. They are the officials at polling places to assure voting is honest and fair, and they have voter registration lists available allowing them to assure election integrity.